Mobile telephony networks (Public Land Mobile Networks, shortly PLMNs) were initially conceived for enabling voice communications, similarly to the wired networks (Public Switched Telephone Networks, PSTNs), but between mobile users. Mobile telephony networks have experienced an enormous spread, especially after the introduction of second-generation mobile cellular networks, and particularly digital mobile cellular networks such as those complying with the Global System for Mobile communications (GSM) standard (and its United States and Japanese corresponding systems). The services offered by these cellular networks in addition to plain voice communications have rapidly increased in number and quality; just to cite a few examples, Short Message Service (SMS) and Multimedia Message Service (MMS) services, and Internet connectivity services have been made available in the last few years.
More recently, 3G mobile communication systems, like those complying to the Universal Mobile Telecommunications System (UMTS), are being deployed, bringing about significantly higher information exchange rates, allowing network operators to offer new services to the mobile users. Moreover new technologies were introduced on the existing 3G standard, such as HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access), in order to accomplish the evolution of new services in terms of throughput made available on the radio interface (up to 14.4 Mbps on the downlink and 5 Mbps on the uplink) and reduced latency. The evolution towards 4G foresees the so called “LTE” (Long Term Evolution) architecture that introduces a completely new radio architecture able to support bit rate up to 100 Mbps connected to a new “All IP based” Core Network architecture, with further reduced latency and mobility managed by IP based protocols.
PLMNs are born as Circuit-Switched (CS) networks and, as such, are more suitable for voice communications than for exchanging relatively large amounts of data. Data communications are better achieved by adopting Packet-Switched (PS) schemes, like in computer networks, particularly the Internet. This remains true also for 3G mobile communications systems, despite their increased communications rate capabilities. The PS domain of the UMTS is constituted by a core network, which is the evolution of the second generation General Packet Radio Service (GPRS) core network, and a radio access network known as the UTRAN (UMTS Terrestrial Radio Access Network). The UTRAN complying with the 1999 release of the standard (so-called “R99”) is able to support PS transmission up to 384 Kbps for the support of person-to-person or content/network-to-person communications, by means of dedicated channel over a radio link.
Usually, in PLMNs, even if provided with a UTRAN infrastructure, the information content is transferred in a Point-To-Point (P-T-P) or unicast mode, upon activation of a session between a User Equipment (UE) and a service provider connected to a packet-switched network, e.g. a server connected to the core network or to the Internet; the activation of such a session involves the setting up of logical and physical connections between the server and the UE. In such a P-T-P communication mode, the radio resources to be allocated for the exchange of data between the network and the UEs depend on the number of different mobile stations simultaneously exploiting the services, even if two or more users take advantage of the same information content at the same time. This limits the possibility of simultaneously accessing available services by several users, unless the radio resources are oversized.
Thus, it is desirable to have the possibility of delivering information contents related to a same service exploitable by two or more users at a time based on a different, Point-To-Multipoint (P-T-M) or multicast/broadcast mode, so as to save the amount of allocated radio resources.
In this respect, the 3GPP (3rd Generation Partnership Project) standardization group is discussing the implementation, both in the GERAN (GSM/EDGE Radio Access Network, wherein EDGE stays for Enhanced Data for GSM Evolution) and in the UTRAN (UMTS Terrestrial Radio Access Network) frameworks, of a new kind of service architecture, named MBMS (Multimedia Broadcast/Multicast Service). Basically, the MBMS targets the simultaneous distribution of information content (particularly, multimedia content) to more than one mobile user from a single serving base station over a common radio resource; this is for instance the case of short clips of sport matches delivered to UEs of mobile users, or of a television channel transmission through the mobile network. In other words, PLMN operators experience the need of proper mechanisms in the network in order to efficiently transport simultaneously the same information content to specified groups of users.
A technique being currently considered for high data rate transmission is HSDPA (High-Speed Downlink Packet Access), which is considered as a “3.5G” system, offering peak data rates up to 10-14 Mb/s, and is expected to be implemented in the next few years. HSDPA is described, inter alia, in technical reports TR 25.308 (e.g. TR 25.308 V.6.3.0) and TR 25.950 (e.g. TR 25.950 V.4.0.1). In general, HSDPA (High-Speed Downlink Packet Access) refers to a data transmission technique for handling a High-Speed Downlink Shared CHannel (HS-DSCH), i.e., a downlink data channel shared between a plurality of users—supporting high-speed downlink packet data transmission—and its associated control channel, in an UMTS communication system. The performance of HSDPA is mainly based on a number of mechanisms that lead to low latency times and high throughput, such as AMC (Adaptive Modulation and Coding), HARQ (Hybrid Automatic Retransmission reQuest), Fast Packet Scheduling.
AMC (Adaptive Modulation and Coding)
In cellular communication systems, the quality of a signal received by a UE depends on number of factors, such as the distance between the desired and interfering base stations, path loss exponent, log-normal shadowing, short term Rayleigh fading and noise. In order to improve system capacity, peak data rate and coverage reliability, the signal transmitted to and by a particular user is modified to account for the signal quality variation through a process commonly referred to as link adaptation. Traditionally, CDMA systems have used fast power control as the preferred method to control the variations of the propagation channel. Adaptive Modulation and Coding (AMC) offers an alternative link adaptation method that promises to raise the overall system capacity. AMC provides the flexibility to match the modulation-coding scheme to the average channel conditions for each user. With AMC, the power of the transmitted signal is held constant over a frame interval, and the modulation and coding format is changed to match the current received signal quality or channel conditions. In a system with AMC, users close to the base station (BTS, or node-B) are typically assigned higher order modulation with higher code rates (e.g. 16 Quadrature Amplitude Modulation, or QAM, with R=¾ turbo codes), but the modulation-order and/or code rate will decrease as the distance from BTS/node-B increases. Generally, each combination of the modulation technique and of the coding technique is called a “MCS (Modulation and Coding Scheme)”: a plurality of MCS levels has been defined, according to the number of combinations of the modulation techniques and the coding techniques.
H-ARO (Hybrid Automatic Repeat reQuest)
H-ARQ can be seen as an implicit link adaptation technique. Whereas in AMC explicit C/I (ratio between useful signal power and noise including interference) measurements or similar measurements are used to set the modulation and coding format, in H-ARQ link layer acknowledgements are used for re-transmission decisions. There are many schemes for implementing H-ARQ, such as Chase Combining (CC) and Incremental Redundancy (IR).
CC (also called H-ARQ-type-III with one redundancy version) involves the retransmission by the transmitter of the same coded data packet. The decoder at the receiver combines these multiple copies of the transmitted packet weighted by the received SNR. Diversity (time) gain is thus obtained. In the H-ARQ-type-III with multiple redundancy versions different puncture bits are used in each retransmission.
IR (or H-ARQ-type-II) is an implementation of the H-ARQ technique wherein instead of sending simple repeats of the entire coded packet, additional redundant information is incrementally transmitted if the decoding fails on the first attempt.
Combining AMC with H-ARQ leads to the best of both worlds: AMC provides the coarse data rate selection, while H-ARQ provides for fine data rate adjustment based on channel conditions.
In order to limit the complexity of the retransmission process, a stop-and-wait (SAW) scheme has been chosen by the 3GPP for implementing the H-ARQ. SAW H-ARQ increases channel utilization efficiency by continuously transmitting a plurality of data packets before receiving an acknowledge (ACK) for the previous packet data. If n logical channels are established between a UE and a Node B, each identified by time or channel number, while one channel is awaiting an ACK o NACK (Negative ACK) the other (n−1) channels continue to transmit.
Fast Packet Scheduling
Examples of schedulers proposed for HSDPA include Round Robin (RR) and Maximum C/I.
The RR scheduler operates by scheduling users based upon their position in a First-In-First-Out (FIFO) queue. Although it provides the least complex operation and the most fairness between users, the UEs channel conditions are not taken into consideration. As a result, users may be scheduled when experiencing a destructive fade, causing the packet to be corrupted.
As an alternative, the Maximum C/I algorithm schedules users when their instantaneous Signal over Interference Ratio (SIR) is the highest amongst all users at the respective base station. This scheduling algorithm ensures that all users are served on a constructive fade, and as a result, has a higher percentage of successful transmissions. Furthermore, the throughput and spectral efficiency is maximized because the highest possible MCS level is used during each transmission. The disadvantage, however, is the lack of fairness between users in the sector.
With regards to the HS-DSCH transport channel, two architectures have been considered by the 3GPP as part of the study item: an RNC (Radio Network Controller)-based architecture consistent with R99 architecture and a Node B-based architecture for scheduling. Moving the scheduling to the Node B enables a more efficient implementation of scheduling by allowing the scheduler to work with the most recent channel information. The scheduler can adapt the modulation to better match the current channel conditions and fading environment. Moreover, the scheduler can implement algorithms in order to exploit the multi-user diversity by scheduling only those users in constructive fades. Thus, it was decided to directly terminate the HS-DSCH channel at the Node B. In particular, the new functionalities of H-ARQ and HS-DSCH scheduling are included in the MAC (Medium Access Control) layer: in the UTRAN, these functions are included in a new entity called MAC-hs located in Node B.
The basic downlink HS-DSCH channel configuration consists of one or several HS-PDSCHs (High Speed Physical Downlink Shared CHannel), combined with a number of separate Shared physical Control CHannels, HS-SCCHs. The set of shared physical control channels allocated to the UE at a given time is called an HS-SCCH set. According to TR 25.308, the number of HS-SCCHs in a HS-SCCH set as seen from the UE's point-of-view can range from a minimum of one HS-SCCH to a maximum of four HS-SCCHs. The UE shall monitor continuously all the HS-SCCHs in the allocated set.
HS-SCCH is used to inform the users on when they are to be served, as well as in order to provide them with information needed for the decoding process, on HS-PDSCH. There is a fixed time offset between the start of the HS-SCCH information and the start of the corresponding HS-PDSCH sub-frame. For each HS-DSCH TTI (Transmission Time Interval), each HS-SCCH carries HS-DSCH-related downlink signaling for one UE. The following information is carried on the HS-SCCH:                Transport Format and Resource Indicator (TFRI), carrying information about the dynamic part of the HS-DSCH transport format, including transport block set size and modulation scheme. The TFRI also includes information about the set of physical channels (channelisation codes) onto which HS-DSCH is mapped in the corresponding HS-DSCH TTI.        H-ARQ-related Information (H-ARQ information), including the H-ARQ protocol related information for the corresponding HS-DSCH TTI and information about the retransmission/redundancy version.        
Furthermore, the HS-SCCH carries a UE identity (UE Id Mask, or simply UE, ID) that identifies the UE for which it is carrying the information necessary for decoding the HS-PDSCH.
In particular, the first part of the HS-SCCH contains the channelisation code set and the modulation scheme for the HS-DSCH allocation, whereas the second part containing the transport block size and H-ARQ related information. Even more particularly, HS-SCCH is organized so that each TTI is subdivided in subframes of three timeslots, having the same length of the HS-DSCH subframes: the first part of the HS-SCCH information (CS and channelisation code set) is sent on the first timeslot, whereas the second part of the HS-SCCH information transport block size and H-ARQ information) is sent on the third timeslot.
US patent application no. 2003/0035403A1 tackles the problem of providing a method for transmitting information shared by all UEs supporting the same HSDPA service so that the UEs can receive the information at the same time, in an HSDPA communication system. According to US 2003/0035403A1, such problem is solved by a method comprising the steps of: upon generation of the common information, transmitting control information including common ID information indicating the common information over a shared control channel (SHCCH); and transmitting the common information over the SHCCH in a TTI (Transmission Time Interval) equal to or after a TTI where the control information is transmitted.